Ubiquitin and the small ubiquitin-like modifier (SUMO), have critical but classically distinct roles in genome stability and responses to environmental toxins. Ubiquitin and SUMO are covalently attached by E3 ligases to target proteins, to regulate the cell's proteome dynamically. Ubiquitin and SUMO pathway dysfunction results in genetic instability and cancer predispositions in humans. We recently discovered the SUMO-targeted E3 ubiquitin ligase family (STUbLs), which ubiquitinate and target sumoylated proteins to the proteasome for degradation. This unanticipated phenomenon requires a revision to the spectrum of SUMO functions. This revised proposal builds upon our discovery and adds new data that directly connects the STUbL mechanism to cancer and genomic stability. We have identified the main mediators of genomic instability in fission yeast STUbL mutants and we will characterize the cellular and molecular mechanisms of STUbLs through these targets. Results from fission yeast will provide a framework for studies conducted on the human and mouse STUbL targets. Notably, these targets place the STUbL mechanism of action at the intersection of cell growth/apoptotic pathways, genomic stability and oncogenesis. Our two Specific Aims focus on defining the molecular mechanisms of STUbL function both in vitro and in vivo. In Aim 1, we will test the hypothesis that STUbLs act as gatekeepers for apoptosis versus DNA repair pathways, by controlling the fates of key SUMO conjugates. In Aim 2, we will address our overall hypothesis that STUbLs are cellular master keys, controlling genome stability through specific and definable protein interaction interfaces. To attain our Aims we will integrate powerful yeast genetics with cutting-edge structural analyses, MudPIT mass spectrometry, biochemistry and mammalian cell biology. The expected results will provide critical mechanistic insights into the genome stabilizing roles of STUbLs including RNF4, and may suggest therapeutic strategies for RNF4- linked human diseases. Additionally, a detailed understanding of STUbL function should yield general implications for responses to genotoxic stress.